Contact



Jan. 10, 1939.

C. B; GWYN, JR

CONTACT Filed Oct. l1, 1955 nveuntor fn/drm" 15. Hwy, .fr

Patentedi Jan. 10,l 1939 UNITED STATES PATENT ori-ica 2,143,315 e ooNTAc'r Application October 11, 1935, Serial No. 44,486

1 Claim.

This invention relates to electrical make-andbreak contacts and more particularly to electrical make-andbreak contacts of refractory materials.

The general objects of the invention are the 5 provision of electrical make-and-break contacts of improved characteristics and of methods of forming and preparing such contacts.

More specically the invention is directed to the provision of contacts in which the possibility l of failure in service is obviated or minimized,

which are capable of carrying and breaking 20 with less grain boundary material than is found in refractory metal contacts of the type hereto fore used.

Still. another object is the provision of a refractory contact in which there is a virtual absence 25 of amorphous grain boundaries.

Other objects of the invention will be apparent from the following description taken in connection with the appended claim.

The present invention comprises the combina 3U tion of elements, methods of manufacture, and the product thereoi brought out and exemplied the disclosure hereinafter set forth, the scope of the invention beiung indicated in the appended claim.

35 While a preferred embodiment of the invention is described herein, it is contemplated that considerable Variation may be made in the method of procedure and the combination of elements without departing from thespirit of the inven- For a fuller understanding of the nature and objects of the invention, as well as for specific fulfillment thereof, reference should be had to the following detailed description taken in connec- 45 tion with the accompanying figures of the drawing, in which:

Figure l is a perspective view of a short length ci contact metal strip prepared in accordance with my invention;

l 5G Figure 2 is a photomicrograph of a longitudinal section through said strip in the plane ABC of Figure l;

Figure 3 is a photomicrograph of a part of the face of a contact punched from said strip;

d is a photomicrograph o a longitudinal section through a strip of metal of a type heretofore used;

Figure 5 is a photomicrograph of a portion of a face of a contact punched from a strip of metal of the type shown in Figure 4; 5

Figure 6 is a photomicrograph of a longitudinal section through a rod of metal of another type heretofore used; and

Figure 'l is a phtomicrograph of a portion of a face of a contact cut from said rod. il)

Electrical make-and-break contacts of tungstenhave generally been made in the past by sawing the contact discs from wrought tungsten rod or by punching the discs from a rolled tungsten sheet or strip. Ater such contacts have been in use, various troubles frequently develop, such as periodic arcing, resulting in transfer of metal from one cooperating contact to theother with consequent pitting of one contact face and building up of projections on the other. Likewise, very rapid deterioration takes place when the contacts are used to control currents only slightly higher than their rated capacity. These contacts frequently give unsatisfactory operation at low starting speeds as well as high operating speeds. Furthermore, these contacts frequently form oxides on the contact faces thus increasing contact resistance. Where there is a considerable transfer of material from one contact to another the actual contact area is reduced and localized with a resultant increase in current density in the restricted area of contact. This condition renders more pronounced the deteriorating action of the arc.

in order to avoid some of these shortcomings it has been necessary, in a number of instances, to have the contact surface tunduly large and the amperage's comparatively low so that the serv ice life of the contact would not be impracticably short. 40 In order that the distinction between this invention and contacts of the prior art may be fully understood it is desirable to make a comparative study of the appearance of the various contact surfaces,- preferably by microscopic inspection. For such examination the contact surface is preferably polished and then lightly etched with a suitable chemical reagent which serves to develop the grain structure and the boundaries between the grains. Figures 2 to 7, inclusive, of 50 the drawing forming a part of this application are reproduced from photomicrographs of the contacts of the present invention and contacts of the prior art. magnified, in each case, by diameters. 55

, According to one prior art method of making contacts a tungsten ingot is rolled into a flat `strip and contacts are formed by punching discs from the wrought tungsten strip thus produced. Microscopic examination of this contact material, illustrated in Figures 4 and 5, shows the presence of very ne grains i3 in the neighborhood of 30,000 per square millimeter, each of which is surrounded and separated from adjacent grains by material characterized by properties different from those of the main lbodyl of the metallic grains. Throughout this specification and in the accompanying claim, such materials will be referred to as material in the grain boundary con-A dition, whether it is produced in the course of grain structure formation and actually defines the surfaces of the several grains, or is produced in the working of the contact metal, as by swaging, rolling. grinding, punching, sawing and the like.

Wrought tungsten is actually fibrous in structure, as is readily apparent from the appearance of Figures 4 and 5, but is also granular in its microstructure, the grains being quite minute and separated i'rom each other by denite boundaries clearly visible under the microscope at higher magnlilcations. 'I'he total length of the grain boundaries in a unit area and the proportion of the surface area of the contacts made up of material in the grain boundary condition is large. In addition the strain-hardening of the material comprising the grains themselves may contribute to the amount of material in the grain boundary condition.

It has been found that the material in the grain boundary condition is much more active physically and chemically than the material at the center of the grains, particularly under conditions induced by electrical make and break contact operation. Whether this be due to greater -concentration of impurities at the grain boundaries or to amorphous condition of the grain boundary material, or to some other cause, is not as yet positively established. But whatever may be its precise nature, this material in the grain boundary condition has properties distinguishably dierent in microscopic appearance and in electrical and chemical activity from those of the grains themselves.

Many observations of make-and-break contacts of tungsten of the types heretofore generally used, have indicated that the effects of deterioration in actual service manifest themselves first and more markedly at the grain boundaries separating the individual grains in the working surface of the contact. It appears that the material in the grain boundary condition has lower resistance to the deteriorating influences imposed under service conditions than has the material comprising the grain proper. It volatilizes or electrically erodes more readily'than the material of the grains themselves, and these facts together with the other properties peculiar to it, appear to be largely responsible for the arc deterioration and other faulty characteristics of contacts heretofore used.

In my ,Patent 1,958,338, there is illustrated and described a tungsten contact structure wherein the grain count has been made as low as possible, thereby reducing the total length of I grain boundaries and the proportion of the contact area covered by material in the grain boundary condition. In a contact of such structure the ratio of material in the grain boundary condition to material of the grains proper exposed on thev working surface of the contact is minimized purely by virtue of increase in the size of the grains. The necessary operation to increase the grain size and reduce the grain boundary condition involves first swaging, drawing, rolling or the like and then retreating the material at temperature approaching its fusion point in a reducing or neutral atmospheresuch as hy; drogen, helium, argon, etc., or in a vacuum.

A further improvement is obtained through the use of the process described in the patent to Joseph A. Weiger No. 1,958,357. An improved interlocked grain structure is obtained by treating the contact material with an ingredient or ingredients which stimulate and accelerate grain growth and reduce to a minimum impurities and material in the grain boundary condition. The inclusion of this ingredient is preferably accomplished by thoroughly mixing with pure tungsten oxide a small amount of sodium silicate. mixture is then fired and the tungsten reduced to the metallic state by heating in a reducing atmosphere such as hydrogen, after which the resulting material is pressed into slugs. The

resultant slugs are baked at a high temperature in a non-oxidizing atmosphere and further heattreated by passing electric current through them for a short period. During the heat treatment the silicon compound reduces the impurities, in-

cluding oxygen, in the tungsten, decreases the amount of material in the grain boundary condition and in addition, this ingredient accelerates and stimulates the grain growth, thereby producing a material with a relatively large grain structure. After performing its desired functions this ingredient is largely volatilized by this heat treatment.

The treated slugs are then hot-swaged into rods, the temperature being accurately controlled to avoid equiaxing the grain structure of the metal'and to insure the production of a strain l worked metal with distorted and interlocked grains. The swaged rods are then heated in a non-oxidizing atmosphere, preferably of hydrogen, by passing an electric current through them, A. the metal being quickly heated to a temperature of about 2400* C., the temperature being held at the maximum value for a short period of time, approximately two minutes, after which the metal is cooled rapidly. This latter heat treatment is performed'to obtain an increase in the size of the grains, the temperature being increased to the maximum value rapidly, such that the grains will not equiax but have the desired distorted and interlocked structure. The contacts are then cut or sawed from the rod in the form of discs. As a result a contact is obtained having a distorted and interlocked grain structure. A section through such a contact is shown in Figure n6 and a portion of its face is shown in Figure 7. The interlocked structure can be seen from the shape of grains i4, Figure 6 and I5, Figure 7. The grain count is less than 2,000 per square millimeter but is considerably greater than the grain count of approximately 29 to the square millimeter obtained by the process of my prior application. The grain count of the particular contact illustrated is in the order of 200' grains per square millimeter.

When it is considered that standard ne grain tungsten contacts such as illustrated in Figures 4 and 5 have a usual count of 20,000 to 80,000 grains per square millimeter, it will be appreciated that the methods referred to in both the above patents have reduced the grain count and apparently The 1 Wise reduced with increased grain size.

At this point, it may be noted that when an ingotvor the like of tungsten is rolled or drawn the granular structure is developed parallel to the longitudinal axis of the drawn or rolled strip or rod. The iibrous appearance is readily apparent in Figures 4 and 5 and a definite orientation ofthe grains is even noticeable in the heattreated material shown in section in Figure 6. In each gure, the longitudinal dimension of the rod from which the photomicrographic sections were taken can be determined by noting in which direction the granular or fibrous structure appears to run.

As previously indicated, contacts having the microstructure shown in Figures 4 and 5 are made by punching the discs from rolled strip of standard, line grained tungsten. In such discs, as is clearly seen from these figures a definite alignment of the grain structure exists of a very marked character. Figure 4, which is a view of a longitudinal section of this ordinary tungsten rod `after being flattened by rolling, shows the laminations 'separated by easily definable grain boundaries running parallel to each other and to the contact surface. This structure is very brittle and actual fracture tests prove the material thus formed to be very easily fractured along the line of cleavage parallel to the contact surface, the tungstenvscaling ofi in layers.

The present invention provides a contact which does not have these undesirable properties. According to the preferred method of carrying out the presentinvention tungsten is mixed with a silicate, a Water soluble silicate such as sodium or potassium silicate being preferred. The desired amounts of these ingredients, (preferably sodium silicate 0.1 to 0.5% the remainder tungsten) are mixed by mechanical or other means and pressed into slugs of desired sizes. These slugs may then be sinterel, preferably at 1200 to 1500 C. They are then heat treated by passing electric current through them to raise the ternperature to 2400 C. to 3200 C., preferably to around 3100" C. The resultant material contains some silicon. It is possible that this silicon may be present as the oxide or may be combined with the tungsten in some Way. The exact state of the silicon at this stage is not, however, known with absolute certainty. The slugs are then swaged into rods of the proper size in accordance with the usual procedure heretofore known in the art for manufacture of metallic rods. Thereafter these rods may be heated electrically or otherwise to a high temperature, preferably within the range 2400n C. to 3200 C., in a neutral or nonoxidizing atmosphere such as hydrogen, helium, nitrogen, argon, etc. or in vacuum for the time required to produce a desired grain size and structure. The time required may be from 1 to 15 minutes. The rods are then cooled rapidly to a temperature which permits their removal from the treating atmosphere without danger of detrimental oxidation.

An important novel step in the process of manufacture of the contacts of this invention, which step may precede or follow the last above described step, as found most desirable in practice, and which brings about an improvement over the contact of Patent 1,958,357, consists in further rolling the rod so as to flatten the same and bring it to the desired thickness for a contact disc.

The rolled material is cleaned, preferably between the nnal rolling operations, using hot caustic soda solution or other cleansers suitable for the purpose. A suitable caustic soda solution is one prepared as a saturated solution at 100 C.

By such a procedure there is obtained a strip l0, a,` section of which is shown in Figure 1, wherein the orientation indicated by lines il is still longitudinal of the strip. The contact discs are stamped out of the strip in such manner that the wide longitudinal face of the strip becomes the working face of the contact (as indicated by circle I2) The orientation of the grain structure then lies generally parallelto the face which acts as the contact surface when in use.l The material appears to have an interlocked plate structure with virtual absence oi visible' grain boundaries. While a considerable reduction in the grain count is obtainedl by the methods of the present invention as in the preceding methods i above referred to, yet the process is greatly simplified by not reducing the grain count to such a great extent as heretofore attempted but obtaining a better resultant product by an alteration in the grain structure and particularly the orientation thereof accompanied by a greater elimination of the material of which the grain boundary condition is formed.

During the fabrication thus described, no swaging of the rod is required prior to rolling, other than that ordinarily performed during tungsten rod Afabrication.

As an example of the steps that may be followed in producing a iiattened strip of the material produced as above described, starting with alloyed rod .125 inch in diameter where a nished contact disc of '.030 thickness is desired, the rolling stages are preferably as follows: First, down to .080 to.085-inch flattened thickness; second,

down to .060 to .065 inch thickness; third, the

flattened rod is cleaned with caustic preferably as cited above; fourth, the rolling then reduces the rod to .040 to .045 fiattened thickness; ith, the rod is again cleaned with caustic; sixth, the rod is` rolled to the final desired attened thickness of .030. The finished rod or strip will then be of the general shape indicated in Figure i.

Contacts are punched from the flattened rod as indicated by circular line l2, the grain extending across the face of the punched contact. The

punched contact discs are then preferably cleaned for several hours in hot caustic soda solution or other cleansing composition and thereafter welded or brazed in conventional manner to the proper types of support for use.

The material fabricated according to the present disclosure is characterized microscopically type similar to that which would result from rolling the material shown in Figures 6 and l rather than merely strain-worked fibrous tungsten of the type illustrated in Figures 4 and 5. The grain count obtained bythe present invention is ordinarily 300 grains per square millimeter on the contact face. The structure seems to be, under microscopic analysis, a mass of interlocking and interwoven plate like grains wherein the grain boundaries (containing amorphous grain boundary material) are of infinitesimal area as comparedto grain area, both in planes parallel to or in angular relation to the contact surface.

'I'he laminated structure developed when prior art fine grained tungsten rod is rolled into strip material of the type shown in Figures 4 and 5 is not present in the tungsten obtained according @by its grain structure which is of an interlocking to the present invention and as indicated in Figures 2 and 3. The contact material of my invention, while described in general as plate-like, lacks a definitely laminated orientation, being neither definitely equiaxed nor cleavable nor columnar nor particularly brous.. The crystals themselves are apparently flattened by the rolling process. The ratio of area of crystal face to volume of crystal is thereby increased.

It is believed that the heating and mechanical working of the material has set up a condition which is conducive to optimum grain growth, whereby the amount of amorphous grain boundary material is held to a minimum. 'I'he inished contact material ordinarily will contain only a small proportion of silicon, usually from less than one-tenth of 1% to one-half of 1%. While the resultant `product cannot be described as a single crystal, yet its excellence from an operating standpoint and its practicability from Va manufacturing standpoint, with ready duplication, renders it equal if not superior to a single crystal.` The infinitesimal area of the grain boundary material as compared to the grain area can be seen in Figures 2 and 3 where very few grain boundaries are visible and such as are discernible are of rare occurrence and extreme thinness as compared with those found in the materials illustrated in Figures 4 to '1, inclusive. This condition of the material including the virtual absence of grain boundaries, is existent throughout the contact (as is apparent from Figure 2) and not merely upon the surface.

Among other advantages obtained by the use of contacts of the present invention as compared with the prior art contacts previously described are the following:

When used on vibrator horns, it has been found that the present contacts will carry approximately 25% more peak current and hold a more uniform tone due to the lessened resistance rise, as compared to the standard punched tungsten contacts formerly used, of a structure as shown in Figure 5.

Formerly it was the practice to use a condenser or resistance in parallel with the contact points to cut down destructive arcing and reduce metal transfer. With the present contacts, these have been eliminated in many horns, and a more uniform tone for a greater period of time is secured than possible under like conditions with standard punched tungsten points.

Also, formerly, due to the wearing of the contacts, the spacing could not be maintained and a very non-uniform tone condition would result. This has been practically eliminated with the contact material of my invention. In the present contacts the erosion or wear is more uniformly distributed over the face of the contacts and is not localized.

The hardness and toughness of the contact as a whole is increased through the processes of this invention.

In the operation of vibrator type horns, workandere ing-at frequencies o! 200 to 500 cycles per second,

depending upon pitch, contacts of the prior art' rapidly disintegrate from electrical effects and also wear out mechanically. Due to the peculiar ,My tests, both in the laboratory and in the field, show that these improvements have been brought about by the distinctive contact structure produced by the processes herein described.

While I have referred particularly to the use of a silicate with the tungsten'it is to bejunderstood that I may use vanadium or titanium compounds as a substitute for the silicate although the handling of these materials is harder to control. Furthermore, some other refractory contact material such as molybdenum or the carbides of tungsten or molybdenum or admixtures thereof, may be substituted for tungsten. Rhenium or its carbide may likewise be made into contacts by the same procedure. Apparently, however,v the use of tungsten with a Water soluble metal silicate produces the most satisfactory contact material.

Furthermore, while l have referred more particularly to the application of my invention to the production of contact discs, such as are used in automobile horns, distributors and ignition systems, the invention is equally applicable to the production of X-ray targets, electrodes forl soluble alkali metal silicate with an oxide of a refractory metal selected from the groups consisting of tungsten and molybdenum, pressing the resultant mixture into a slug, heat treating the slug at a temperature Aof 2400 degrees C. to 3200 degrees C., hot swaging the slug into a rod, heat treating the rod at a temperature of 2400 degrees C. to 3200 degrees C., whereby a rod is produced having the refractory metal present in the form of substantially equiaxed, interlocking grains,

separated by grain boundaries containing amorphous grain boundary material in an amount insuicient to prevent interlocking of said grains, thereafter successively rolling and cleaning said rod in a series of operations to reduce said rod to sheet form, and then punching a contact disc vfrom the resultant sheet, said contact disc hav- 

